Centralisers are well known in the field of oil and gas drilling and production. Centralisers are used to maintain a minimum stand-off or radial distance between the inner surface of a bore of a well, and a device being deployed (usually a tubular or string of tubulars) within the bore. Often, the bore can be lined, for example, with tubular casing or liner, and the string of tubulars is centralised within the bore of the casing or liner, but centralisers can also be used in un-lined bores.
The function of the centraliser is to maintain a consistent radial spacing or stand-off between the outer surface of the device in the bore and the inner surface of the bore, so that the annulus between the device and the bore has a generally consistent radial dimension. This is desirable for a number of reasons. In certain operations in which centralisers are used, for example in completion operations, the annulus between a tubular string and the inner surface of the bore is filled with cement, and it is desirable that the layer of cement surrounding the tubular has a generally consistent radial dimension along the length of the tubular. Therefore, centralisers are deployed between the outer surface of the tubular and the inner surface of the bore at intervals along the tubular in order to maintain the stand-off so that the layer of cement formed in the annulus has a generally consistent radial depth along the length of the tubular.
Centralisers can be of the solid body type, being cast or otherwise formed in a single piece. An example of this type of centraliser is described in our earlier granted patent U.S. Pat. No. 5,797,455, the disclosure of which is incorporated herein by reference. Centralisers can also be of the spring bow type, having end collars with resilient strips of metal extending radially outwards in the form of bows between the collars. The bows are compressed and resiliently energised when the centraliser is inserted into the bore, and are designed to remain in compression when in the bore to hold the tubular in or near to the centre of the bore. Examples of this type are described in EP0196339, CN2119492 and in US2011/0030973, the disclosures of which are incorporated herein by reference, and which are useful for understanding the invention.